The present invention relates to a magnetic recording medium adapted to perpendicular magnetic recording.
The perpendicular magnetic recording system is a means for magnetic recording in a direction perpendicular to the film surface of a magnetic recording medium, and is suitable for increasing the magnetic recording density since the demagnetizing field in every bit is small in high density magnetic recording. Magnetic recording media usable for this purpose include those using a thin film of a Co-based alloy such as Co-Cr, Co-V, Co-Mo, Co-W, Co-Ru, Co-Re, Co-O, Co-Cr-Rh, Co-Cr-Ru, or Co-Ni-O. The Co-based alloy constituting the thin magnetic film is characterized in that it has a close-packed hexagonal (h. c. p.) lattice structure and consists of fine crystal grains liable to be oriented in respect of their c-axes. In order to improve the magnetic recording characteristics, not only must c-axis dispersion of the thin Co-based allov film be decreased, but also the size of fine columnar crystal grains must be controlled. As discussed by Iwasaki et al. in "Perpendicular Magnetic Recording with Composite Anisotropy Film", IEEE, Trans. Magnetics, MAG-15, 1456 (1979), provision of a thin film layer of a soft magnetic material such as Permalloy under the thin Co-based alloy film is effective in improving the magnetic recording sensitivity.
The perpendicular magnetic recording medium used today comprises a non-magnetic substrate such as a plastic film of polyimide or polyethylene terephthalate or an Al or glass plate, and a thin Co-based alloy film directly deposited thereon or formed with a thin film layer of a soft magnetic material such as Permalloy therebetween for the purpose of improving the magnetic recording sensitivity. In the case of a thin Co-based alloy film directly deposited on the non-magnetic substrate, the c-axis dispersion (.DELTA..theta..sub.50 ) thereof is about 8.degree. to 9.degree., and the columnar crystal grain size is not controlled at all with quite irregular grains, with the result that it is poor in the performance characteristics as a high density magnetic recording medium. With a view to improving the c-axis dispersion of the thin Co-based alloy film as the above-mentioned magnetic layer, Sugita et al. has proposed a process comprising depositing Ti on a non-magnetic substrate by the vacuum evaporation technique, and providing thereon a magnetic layer or forming a thin permalloy (Ni-20 wt % Fe alloy) film layer between the thin Ti film layer and a magnetic layer as provided above (Japanese Patent Laid-Open No. 77,025/1983). This process can provide a thin Co-based alloy film having a c-axis dispersion .DELTA..theta..sub.50 of about 6 to 8.degree.. However, a higher value of .DELTA..theta..sub.50 is required of a further higher density magnetic recording medium, and a thin film comprising columnar crystal grains so controlled as to have adequate sizes must be provided in order to improve the magnetic recording characteristics. In the case of a thin Co-based alloy film as the magnetic layer under which a thin film layer of a soft magnetic material is provided, the c-axis dispersion thereof is poorer than that of the Co-based alloy film directly provided on the non-magnetic substrate, and a grave difficulty is encountered in controlling the size of columnar crystal grains in the thin film.